The present invention relates to a method for controlling the transmission power in a radio system, and to a corresponding radio system, in particular a corresponding mobile radio system.
Transmission power control in mobile radio systems is an important performance feature in order to be able to suppress possible interference between the individual connections and, hence, to allow the capacity and quality of the connections to be improved, as well as to be able to reduce the mean transmission power, to achieve the best possible matching to the requirements and to allow losses via the transmission channels to be at least partially compensated for.
To this end, the signal transmitted by a transmitter is evaluated at the receiving end in the mobile radio system in order to allow information to be produced as a function of this for power control, and to allow this information to be transmitted to the transmitter, which then adjusts the transmission power in accordance with the power control information.
In order to explain the principle of power control in more detail, FIG. 2 shows the communication between a base station 1 and a mobile station 2 in a mobile radio system. A connection from the base station 1 to the mobile part 2 is referred to as the downlink or forward link connection, while a connection from the mobile part 2 to the base station 1 is referred to as the uplink connection, or reverse link connection. For power control in the downlink, the respective received signal is evaluated in the mobile station 2, and power control information is produced as a function of this and is sent back to the base station 1, so that the base station 1 can adjust the transmission power appropriately. In order to control the uplink, the received signal is evaluated in the base station 1, where the power control information is produced and the mobile station 2 is instructed to carry out power matching.
The power control information is, in this case, transmitted within a predetermined frame structure, depending on the respective mobile radio system.
FIG. 3 shows an example of the frame and timeslot structure for a downlink connection in a mobile radio system operated using a code division multiple access method (CDMA). The frame and timeslot structure shown in FIG. 3 corresponds, in particular, to a UMTS mobile radio channel (Universal Mobile Telecommunications System), which is also referred to as DPCH (Dedicated Physical Channel) in accordance with the current state of UMTS standardization. UMTS is the designation for third-generation mobile radio systems, with the aim of a worldwide, universal mobile radio standard. According to the UMTS mobile radio standard, the WCDMA method (Wideband Code Division Multiple Access) is intended for use as the multiple access method.
The frame structure shown in FIG. 3 and with a duration of 720 ms includes, in particular, 72 frames 3 of identical construction and having a frame duration of 10 ms, with each frame, in turn, having 16 timeslots 4, with a timeslot duration of 0.625 ms. Each timeslot 4 includes information split between a logical control channel (DPCCH, Dedicated Physical Control Channel) and a logical data channel (DPDCH, Dedicated Physical Data Channel). The DPCCH section includes a pilot bit sequence 5 and TPC information (Transmitter Power Control) 6 and TFI information (Transmitter Format Identifier) 7. The DPDCH section includes user data bits 8. The structure shown in FIG. 3 can be found, for example, in the document ETSI STC SMG2 UMTS-L1: Tdoc SMG2 UMTS-L1 221/98.
The pilot bit sequence 5 is used for estimating the channel impulse response during a training sequence, and corresponds to a known bit pattern. The receiver can determine or estimate the channel impulse response of the mobile radio channel by comparing the received signal with the known pilot bit sequence.
The TFI information 7 is used for format identification for the respective receiver. The TFI bits are protected, according to the present WCDMA Standard, via a specific coding method, and are distributed over an entire frame (time duration 10 ms) by interleaving. If the TFI information 7 in each timeslot includes, for example, three bits b4 . . . b5 as shown in FIG. 3, there are a total of 3*16=48 TFI bits per frame, which includes 16 timeslots, and these bits are coded via a bi-orthogonal coding method.
The TPC information 6 represents the command, produced by the receiver and transmitted to the transmitter, to adjust the transmission power. To do this, the received power in the receiver or the signal-to-noise ratio in the received signal is compared with a predetermined reference value, and the value for the power adjustment command is determined as a function of the discrepancy. As such, if the received power exceeds the reference value, a command is produced to reduce the transmission power, while a command to increase the transmission power is produced if the received power is less than the predetermined reference value. Thus, depending on the comparison result, the receiver transmits a digital or binary adjustment command to the transmitter. In this case, a command to increase the transmission power (power up command) is coded with a 1, while a command to reduce the transmission power (power down command) is coded with a 0. In each case, the adjustment command is transmitted to the transmitter after appropriate modulation. According to the WCDMA Standard for UMTS mobile radio systems that is currently under discussion, the transmission is carried out via QPSK modulation (Quadrature Phase Shift Keying), by which the binary 1 or 0 are changed to respective values −1 and +1, followed by the power control signal being spread.
The power control information, thus, generally includes only one bit, which indicates whether the transmission power should be increased or reduced at the transmission end. In order to allow this bit to be transmitted with a sufficiently low error probability, the bit is transmitted repeatedly. The TPC information shown in FIG. 3 in consequence, for example, includes three bits b1 . . . b3, which are transmitted successively with identical information contents. However, the power control information also may include a different number of bits; for example, more bits.
Relatively powerful coding methods which are known per se and via which it would be possible to achieve a better error probability are not used since the TPC bits will need to be evaluated immediately in the receiver of the TPC information in order to allow the transmission power to be readjusted appropriately without delay. According to the prior art, the TPC bits are not coded together with other bits or data and also can not be distributed over a relatively long time period, for example over an entire frame, which is referred to as interleaving.
However, there is a requirement for the TPC bits to be transmitted correctly with as high a reliability level as possible in order to avoid the transmitter incorrectly or not reliably receiving the corresponding power adjustment command.
The document ETSI SMG2 L1 Expert Group, Tdos SMG2 UMTS-L1 736/98, Espoo, Finland, Dec. 14-18, 1998, “Soft TPC Interpretation for Improved Closed Loop Power Control” discusses the reliability of the power adjustment command transmitted to the transmitter, and adjustment in as optimum a manner as possible of the value of the power adjustment command as a function of the reliability of its reception. In this case, the authors indicate that the value of the power adjustment command should be chosen depending on the function tanh (Λ/2), where Λ represents the reliability of the power adjustment command in the form of a log-likelihood distribution.
The present invention is directed toward providing an improved method for controlling the transmission power in a radio system, in particular in a mobile radio system, as well as a corresponding radio system, by which the reliability of transmission of the power control information can be improved.